Non-corrosive double-walled tube and proces for making the same

ABSTRACT

A process for brazing non-ferritic steel surfaces such as nickel chromium or stainless steel to which a suitable brazing alloy such as copper has been mechanically attached which includes the steps of instantaneously elevating the surface of the stainless steel to a brazing temperature while maintaining the material in a humidified gaseous atmosphere consisting essentially of a non-reactive carrier gas and a reactive gas present in sufficient concentrations to achieve fluxing; maintaining the surface temperature of the steel for an interval sufficient to permit fusion between the selected metal and the non-ferritic steel surface; after metal fusion has been achieved, allowing the resulting fused metal material to cool to a first lowered temperature in a controlled non-oxidative atmosphere at a rate which retards the formation of fine-grained steel crystals in the metal; and after reaching a metallurgical transformation point, rapidly cooling the fused metal in a controlled atmosphere to a temperature below which the brazing metal is not reactive with oxygen. 
     This process can be employed successfully in manufacturing unique, seamless double-walled tubing which is relatively flexible and highly corrosion resistant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to corrosion-resistant double-walled tubes andprocesses for manufacturing such tubes. Double-walled tubes such asthose produced by the process of the present invention are suitable fora variety of uses such as in automotive brake lines. This invention alsorelates to a process for brazing a selected metal to a non-ferriticsteel surface.

2. Discussion of the Relevant Art

Safety standards in the automotive industry dictate that criticalelements such as automotive brake lines be resistant to leakage,puncture and corrosion. In order to achieve these objectives,double-walled tubes for brake lines have been adopted as the industrystandard. Such a double-walled tube consists of at least two thicknessesof a breakage-resistant metal material having sufficient properties towithstand fatigue due to prolonged vibration. The double-walled tubeemployed in automotive vehicles is generally referred to as "seamless",meaning that there is no seam extending the radial length of the tubewall, from the inner diameter of the tube to the outer diameter. Thus,the possibility of leakage at any joined seam is essentially eliminated.The material of choice, up to this point, has been carbon steel due toits inherent flexibility, strength and mechanical durability. A suitablebrazing material such as copper or various copper alloys is plated overthe surface of the carbon steel prior to formation to permit theultimate formation of a seamless joint.

A drawback of carbon steel is its susceptibility to corrosion. In anattempt to eliminate this problem, carbon steel brake line tubes havebeen plated with a variety of corrosion-resistant materials, the mostcommon of which is zinc. Zinc is plated onto the carbon steel surface ofthe brake line tube in a post-manufacturing process. In order to achievesufficient corrosion resistance, plating thicknesses as great as 25microns have been employed. Unfortunately, the zinc-plated surface issusceptible to cracks and chips due to road hazards and continuedprolonged vibration. This leads to corrosion and, ultimately, to leakingof the brake line tube.

In order to alleviate this problem, zinc-plated carbon steel tubes havebeen further coated with high-strength polymers such as polyvinylfluoride. Polyvinyl fluoride coatings can also crack and chip, andultimately lead to corrosion problems. Additionally, brake lines coatedwith polyvinyl fluoride are difficult to dispose of or recycle once thevehicle has reached the end of its useful life.

Ideally, brake lines would be manufactured from a suitable, inexpensivenon-corrosive material. However, corrosion-resistant metals such asnickel-chromium (stainless) steel are not amenable to double-walled tubemanufacturing processes. Great difficulties have been encountered inimparting a copper overlay to a stainless steel surface, and it has beenwidely held that copper-plated stainless steel could not bemetallurgically brazed.

Additionally, the forming processes for manufacturing a continuousseamless double-walled tube require the use of lubricating compounds ormaterials, such as milling oils, which adhere to the surfaces of themetal and interfere with achieving a uniform 360° metallurgical braze.Removal of these contaminants prior to brazing is imperative butdifficult.

Thus, it would be desirable to provide a process for manufacturingseamless double-walled tubing in which contaminating lubricants can beremoved or rendered harmless prior to metallurgical brazing. It is alsodesirable to provide a process in which a highly corrosion-resistantbase metal such as non-ferritic or stainless steel can be successfullyand economically employed. It is desirable to provide a process in whicha selected metal alloy can be successfully metallurgically bonded to anon-ferritic steel surface. It is further desirable to provide acorrosionresistant double-walled seamless tubing suitable for use in themanufacture of automotive brake lines.

SUMMARY OF THE INVENTION

The present invention encompasses a process for brazing a selected metalalloy to a non-ferritic steel surface in which the selected metal alloyis plateable on the steel surface. In this process, the temperature ofthe non-ferritic steel is raised from a first temperature to a secondelevated temperature and maintained at that second elevated temperaturefor an interval sufficient to achieve fusion between the selected metalalloy and the non-ferritic steel surface. The first temperature is lessthan or essentially equivalent to the volatilization temperature oflubricating materials adhering to the steel surface, if any. The secondelevated temperature is sufficient to trigger fusion between theselected metal alloy and the non-ferritic steel surface. The secondtemperature elevation occurs in a humidified gaseous atmosphere whichconsists essentially of a non-reactive carrier gas and a reactive gassuitable for and in sufficient concentrations to achieve and promotefluxing.

The non-ferritic steel is maintained in contact with the humidifiedgaseous atmosphere at or above the second temperature for an intervalsufficient to permit fusion between the selected metal alloy and thenon-ferritic steel surface. After metal fusion has been achieved, theresulting fused metal material is allowed to cool to a first loweredtemperature in a controlled non-oxidative atmosphere at a rate whichmaximizes the temperature at which metallurgical transformation of thenon-ferritic steel from an austenitic to a pearlite phase occurs.

After the metallurgical transformation point has been achieved, theresulting material can be rapidly cooled in a controlled atmosphere to atemperature below which the selected metal alloy is no longer reactivewith oxygen. If desired, the resulting metal material can be furtherquenched in a suitable aqueous medium.

Before the brazing process is begun, the process of the presentinvention can include the additional optional step of removing anyvolatile contaminants remaining on the metal surface imparted thereduring any metal deformation steps. In the process of the presentinvention, this preferably comprises the step of elevating the surfacetemperature of the non-ferritic steel from a preliminary temperature tothe volatilization temperature. The preliminary temperature may beambient or any intermediate temperature substantially below thevolatilization temperature. The temperature elevation step proceeds inthe presence of a non-oxidative atmosphere at a rate sufficient toinitiate an essentially instantaneous volatilization of volatilizablesolvents and carriers present in the lubricating material.

The process of the present invention can be successfully employed in aprocess for producing a noncorrosive seamless double-walled tube. Alsoincluded in the present invention is a non-corrosive, seamlessdouble-walled tubing suitable for use in automotive brake lines.

BRIEF DESCRIPTION OF THE DRAWING

In the present description, reference is made to the following drawingin which FIG. 1 is a schematic view of the process of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is predicated on the unexpected discovery that aselected metal alloy such as copper or silver previously plated on anon-ferritic steel surface can be successfully fused to that surface ina brazing process. This brazing process permits the fusion of the doublewalls to one another, thereby producing a continuous, seamlessnon-corrosive metal tube.

The present invention is a process for brazing a selected metal alloy toa non-ferritic steel surface. The brazing process of the presentinvention comprises the following steps:

rapidly raising the temperature of the non-ferritic steel from a firsttemperature to a second elevated temperature sufficient to triggerfusion between the selected metal alloy and the non-ferritic steelsurface, the temperature elevation occurring in a humidified gaseousatmosphere consisting essentially of a non-reactive carrier gas and areactive gas suitable for and in sufficient concentration to achievefluxing;

maintaining the non-ferritic steel in contact with the humidifiedgaseous atmosphere at the second temperature for an interval sufficientto permit fusion between the selected metal alloy and the non-ferriticsteel surface;

once metal fusion has been achieved, allowing the resulting fused metalmaterial to cool to a first lowered temperature in a controllednon-oxidative atmosphere at a rate which maximizes the temperature atwhich metallurgical transformation of the non-ferritic steel from anaustenitic to a pearlite phase occurs; and

after reaching the metallurgical transformation point, continuingcooling of the fused metal material in a controlled atmosphere to atemperature below which the selected metal alloy is not reactive withoxygen.

The humidified gaseous atmosphere employed in the first step ispreferably a mixture of nitrogen with sufficient hydrogen to achieve andmaintain fluxing. A suitable volumetric concentration of hydrogen wouldbe readily discernable by one reasonably skilled in the art. In thepreferred embodiment, it has been found that concentrations of hydrogenbetween about 50% and about 75% by volume can be successfully employed.The process of the present invention is to be construed as encompassingfunctional equivalents of the described gases.

The term "non-ferritic steel" as used herein is generally defined as nonmagnetic, nickel chrome stainless. In the preferred embodiment, thenon-ferritic steel is a stainless steel consisting essentially of iron,chromium, nickel, manganese, silicon, and carbon. The amount of carbonis, preferably, limited to amounts no more than 0.03% by weight. Anillustrative example of one such non-ferritic steel is set forth inTable 1.

                  TABLE 1                                                         ______________________________________                                        TYPICAL ANALYSIS OF A NON-FERRITIC STEEL                                      Element       Percent                                                         ______________________________________                                        Carbon        0.03                                                            Manganese     7.00                                                            Silicon       0.50                                                            Chromium      16.75                                                           Nickel        5.00                                                            Nitrogen      0.07                                                            ______________________________________                                    

The selected metal alloy is one capable of being uniformly deposited onthe surface of the non-ferritic steel. The deposition process may be anysuitable mechanical, chemical, or electrochemical process which willpermit permanent or, at the minimum, semi-permanent mechanical adhesionof a selected metal alloy to the non-ferritic steel surface. Thepreferred selected metal alloys are alloys of metals such as copper,silver or any other suitable alloy. Additionally, non-alloyed metalssuch as copper, silver or any other suitable metal can also besuccessfully used in the process of the present invention.

The deposition process is, preferably, an electroplating process whichcan be employed successfully on non-ferritic or stainless steel whichhas been prepared by a Woods-nickel strike. The Woods-nickel strikeimparts a Woods-nickel composition to the surface of the stainlesssteel. The Woods-nickel composition will mask the existingnickel-chromium oxides to permit copper plating. The plated surface is,then, rendered suitable for subsequent brazing procedures.

Where the objective is the formation of double-walled tubing, theelectroplateable brazing alloy is deposited on the non-ferritic steelsurface prior to formation of the double-walled tubing. Thedouble-walled tubing can be rolled and formed by any conventionalmethod. In order to achieve 360° brazing around the entire tube surface,the subsequent brazing steps of the present invention are employed.

In the preferred embodiment, in order to permit more effective brazingaction, lubricating materials applied to the plated non-ferritic steelprior to formation of the continuous tube may be removed. Thelubricating materials commonly employed in metal formation processessuch as those in which a continuous metal tube is formed generallycontain carbon or graphite materials suspended in a variety ofvolatilizable solvents and carrier materials. During conventionalbrazing operations, these materials are sintered into a carbonaceousmaterial which inhibits brazing action. Without being bound to anytheory, it is believed that the brazing inhibition is due to theinterposition of the carbonaceous material between layers ofnon-ferritic steel to be brazed. The carbonaceous material acts as aninsulating material inhibiting suitable heat transfer.

In the process of the present invention, the formed metal material exitsformation machinery at an essentially ambient preliminary temperaturewith lubricating materials adhering thereto. The surface temperature ofthe non-ferritic steel is rapidly elevated from this preliminarytemperature which is substantially below the volatilization temperatureof the solvents and carriers present in the lubricating material to atemperature equal to or above the temperature at which volatilizablesolvents and carriers present in the lubricating material willexperience essentially instantaneous volatilization. It will beappreciated that this temperature elevation is essentiallyinstantaneous. This unique, essentially instantaneous, temperature riseis described herein as a "shock heating" of the metal surface. It hasbeen found that a shock heat elevation to a temperature equal to orabove 900° F. will achieve the essentially instantaneous volatilizationdesired. In order to prevent undesired oxidation of the selected metalalloy plating, the shock heating step occurs in a non-oxidative gaseousatmosphere. The gaseous material is preferably an anhydrous non-reactivegas such as one selected from the group consisting of nitrogen,hydrogen, carbon monoxide, and mixtures thereof. The non-oxidativegaseous atmosphere permits and promotes the volatilization of thevolatilizable solvents and carriers present in the lubricating compound.In the preferred embodiment, the non-oxidative gaseous atmosphere isnitrogen. However, functional equivalents of nitrogen are contemplatedand considered within the scope of this invention.

Because the volatilization is essentially instantaneous, the solventsand carriers volatilize in a manner which physically drives them fromthe surface of the prepared metal. Where two sheets of metal overlay oneanother, such as in double-walled tubing, this process of rapid shockheating eliminates volatilizable solvents and carriers interposedbetween the two respective layers of non-ferritic steel. Heretofore, ithas been almost impossible to completely eliminate such contaminantswithout employing complex mechanical scrubbing or removal operations. Itis to be understood that in certain situations such as the formation ofdouble-walled tubing, even such mechanical scrubbing is impossible.However, the shock heating process of the present invention permitsremoval of volatilizable solvents and carriers, thereby insuring auniform 360° brazing operation.

Without being bound to any theory, it is believed that the shock heatingprocedure triggers an almost explosive volatilization of solvents andcarriers in the lubricating material. When such shock heating isemployed with double-walled tubing, the explosive force of thevolatilization initiates a micro-expansion between the two respectiveoverlaying walls. The gap between the walls created by themicro-expansion process permits the escape of volatilizable solvents andcarriers. Carbon dust remains as a residue after this step is completed.The residual carbon dust does not interfere with subsequent brazingprocedures. The dust can remain on the interior and various surfaces ofthe tubing until the brazing operation is completed. Any residual dustcan be blown from the exposed surfaces upon completion of the tubeformation process. In the preferred embodiment, the volatilizationtemperature is above the volatilization point of the solvents andcarriers but below any metallurgical phase transformation point for thenon-ferritic steel. This range is between about 800° F. and about 900°F.

Once volatilizable solvents and carriers have been removed from themetal surface, brazing procedures can proceed. In order to promotebrazing, it is necessary to substitute the non-oxidative gaseousatmosphere used in the elevation step with an atmosphere which willsupport fluxing. In the preferred embodiment, the atmosphere forsupporting fluxing is a mixture of nitrogen and hydrogen which has beenhumidified and has a dew point greater than about -42° F. In thepreferred embodiment, the substitution of atmospheres may occur in anyconvenient manner after the solvents and carriers have been volatilized,the non-ferritic steel can be exposed to ambient temperature for a briefinterval during the exchange of gaseous atmospheres. Once this has beencompleted, the temperature of the non-ferritic steel is raised rapidlyfrom the volatilization temperature to a second elevated temperaturesufficient to trigger fusion between the selected metal or metal alloyand the non-ferritic steel surface. The term "fusion" as used herein isdefined as the existence or establishment of a metallurgical bondbetween two dissimilar metals. This rate of temperature elevation is asrapid as possible to approximate or achieve essentially instantaneoustemperature rise. This phenomenon is a second shock heating of the metalmaterial.

The second elevated temperature to which the surface of the non-ferriticsteel is elevated is a sufficient amount higher than the liquidustemperature of the selected metal plated on the non-ferritic steel totrigger and maintain the fusion process. "Liquidus temperature" isdefined herein as the temperature at which a metal or metal alloy beginsto enter its molten state. In the preferred embodiment, where copper isemployed, the liquidus temperature of the copper is 1,980° F. The secondelevated temperature is, preferably, at or above between 2,000° F. to2,050° F. The upper maximum for the second elevated temperature isdetermined by both the properties of the nonferritic steel and selectedmetal material employed. Ideally, the upper temperature is limited to apoint below thermal degradation or melting of the steel substrate and/orthe degradation point of the selected metal material.

The fusion process triggered in the process of the present invention is,preferably, brazing. In the preferred embodiment, the metal material iselevated to the brazing temperature in as rapid a manner as possible. Asdescribed previously in conjunction with the solvent volatilizationstep, the metal is once again "shock heated" to produce a temperaturerise from a point at or below the volatilization temperature of 900° F.to a point at or above the brazing temperature of between about 2,000°F. to about 2,050° F. This temperature elevation rate is sufficientlyrapid to initiate brazing. The temperature rise is essentially"instantaneous". "Instantaneous temperature rise" as defined hereinoccurs in an interval no less than 400 degrees/second. This contrastssharply with conventional brazing procedures in which the temperature isbrought up to the brazing point in a steady controlled manner. Withoutbeing bound to any theory, it is believed that the "shock heating" helpsto initiate opening of the crystal grains present in the nonferriticsteel surface to permit a brazing metal such as copper to penetrate intothe surface.

Once the temperature of the non-ferritic steel has been elevated to thefusion temperature, this temperature is maintained for an intervalsufficient to achieve the formation of a metallurgical bond between theselected metal material and the non-ferritic steel surface. This "heatsoak" phase can be defined as the time at which the material is held atthe appropriate fusion temperature to permit formation of ametallurgical bond between the selected metal material and the steelsurface. Without being bound to any theory, it is believed that this"soak time" continues the opening of the fine grain structure of thesurface of the stainless steel initiated during the second shock heatphase permitting the bond to form. At temperatures above 2,000° F.during the heat soak phase, there is a notable precipitation of carbon;triggering and indicating a change from a martensitic to an austeniticsurface. Without being bound to any theory, it is believed that thisphenomenon may be necessary to achieve bond formation between a selectedmaterial such as copper and non-ferritic steels such as nickel chromiumsteel.

During the heat soak phase, the metal is maintained in a humidifiedgaseous atmosphere similar or equivalent to that employed in the "shockheat" phase. Thus, the humidified gaseous atmosphere consistsessentially of a nonreactive carrier gas and a reactive gas suitable forand in sufficient concentrations to achieve fluxing. In the preferredembodiment, the humidified gaseous atmosphere contains between about 50%and about 75% by volume hydrogen in nitrogen with trace amounts ofwater. Humidification of the gaseous atmosphere may be achieved bybubbling a hydrogen stream through water prior to admixture withnitrogen.

After metal fusion has been achieved, the resulting fused material ismaintained in the humidified gaseous atmosphere and permitted to cool toa first lowered temperature greater than the austenitic phasetransformation temperature of the non-ferritic steel for an intervalsufficient to help maximize the temperature at which subsequentmetallurgical transformation of the non-ferritic steel from anaustenitic phase to a pearlite phase occurs. In the preferredembodiment, the fused metal material is maintained at this first loweredtemperature for about four to eight seconds. This temperature is,preferably, greater than about 1,250° F. After reaching this firstmetallurgical transformation associated with a temperature of about1,250° F. and a time interval of 4 to 8 seconds, the fused metal can berapidly cooled in the controlled atmosphere to a second loweredtemperature below which the non-ferritic metal enters the pearlitephase. In the preferred embodiment, this second lowered temperature isabout 950° F. Without being bound to any theory, it is believed thatthis rapid cooling to the second lowered temperature is analogous to ametallurgical freezing. The term "freezing" as used herein defines aprocess which promotes a coarse grained crystal lattice structure in thenon-ferritic steel. It is believed that this coarse grain structureimproves the malleability of the resulting material.

After reaching this second lowered temperature, the fused metal can becooled at a controlled rate in a nonoxidative atmosphere to a thirdlowered temperature below which the selected metal is not reactive withoxygen. Where copper is employed as the selected metal material, thisthird lowered temperature is below about 500° F. and preferably betweenthe temperatures of about 350° F. and 500° F. Below this temperature,the selected brazed material, such as the copper, is not reactive withoxygen. This prevents undesired discoloration of the copper surface. Atthis point, the material can be safely removed from the controlledenvironment with little or no fear of oxidation or discoloration.

In order to facilitate ease of handling of the continuous double-walledtubing or other metal material, the material can, finally, be quenchedin a suitable aqueous medium.

In order to further illustrate the process with regard to producingnon-corrosive, double-walled tubing, reference is made to FIG. 1 whichschematically depicts a tube forming process and line incorporating theprocess of the present invention.

In FIG. 1 is a production line 10 having a pay-off reel 12 whichcontains strips of non-ferritic steel with the selected metal beingoverlaid and mechanically, chemically or electrochemically attached tothe non-ferritic steel surface.

The continuous sheet of non-ferritic steel is, preferably, between 0.25and 0.35 millimeters thick and has a width suitable for producing adouble-walled tube of an appropriate diameter. The length of thecontinuous strip is determined by handling constraints and requirementneeds. The continuous strip (not shown) is advanced to a suitable rollform milling machine 14 which sequentially produces unbrazed, unsealeddouble-walled tubing. The tubing is immediately advanced to avolatilization station 16 which includes means for removing volatileportions of lubricating material employed during the milling steps (notshown).

In the preferred embodiment, the removing means employed in thevolatilization station is a heating means which comprises a source 18for anhydrous nitrogen gas and a heater (not shown). The heater can beany suitable heating means such as a resistance heater, an inductionheater or a muffle furnace. In the preferred embodiment, an inductionheater is employed. The induction heater is configured to permit theelevation to the volatilization temperature at or above 900° F. at arate and for a period sufficient to permit effective volatilization ofsolvents and carriers employed in lubricating material employed duringthe milling process. In the preferred embodiment, as set forth in FIG.1, any particular section of the continuous double-walled tube isexposed in the volatilization station 16 for a period between about 30seconds and about five minutes. The tubing is, then, conveyed through aconduit (not shown) to the brazing station 20 upon exiting thevolatilization station 16. The conduit and volatilization station 16 areequipped with suitable means for venting the volatilized solvents andintroduced nitrogen gas in a suitable manner (not shown).

The brazing station 20 consists of heating means for rapidly rising thesurface temperature of the continuous tube to an elevated temperaturesufficient to trigger fusion between the non-ferritic steel and theselected metal layers thereon. Also included are means for providing ahumidified gaseous atmosphere within the brazing station. The heatingmeans can be either a resistance or an induction heater or any othersuitable heater capable of essentially instantaneously raising thesurface temperature of the nonferritic steel to a temperature at orabove the brazing temperature for the selected metal layered thereon(not shown). The gas preferably employed is composed of a predominantlynitrogen atmosphere containing sufficient hydrogen to achieve andmaintain fluxing. In the preferred embodiment, the gas is humidified bybubbling the hydrogen through a suitable bubbling tank (not shown). Thegas is supplied in any conventional manner such as from gas bank 22 inFIG. 1.

Once the material has been shock heated in the brazing station 20, it isconveyed to a suitable heat soak station 24. The heat soak station 24can be any type of heater capable of maintaining the double-walled tubeat a temperature at or above a brazing temperature of about 2,050° F. Inthe preferred embodiment, the heat soak station 24 is an elongatedmuffle furnace. The heat soak station 24 is also supplied with thehumidified gaseous mixture of hydrogen and nitrogen from the gas bank22.

Upon exiting the heat soak station 24, the continuous metal tube hasbeen fused into its leak-proof, double-walled state. At this point, itcan be conveyed through a water-cooled jacket 26 to provide a controlledcool-down phase in which the elevated temperature is maintained at orabove about 1,250° F. for a period sufficient to control grain size andstructure to provide larger grain size for greater flexibility of theresulting tube. In general, each particular section of the continuoustube is exposed to the controlled cool-down phase for a periodsufficient to maximize the temperature at which the non-ferritic metaltransforms from austenitic through its transition phase into itspearlite phase. The initial controlled cool-down occurs immediatelyafter the section of the continuous double-walled metal tube exits theheat soak station 24. This phase proceeds for a period of about three tosix seconds. The continuous metal tubing is maintained in a controlledatmosphere and is, then, quickly cooled through its isothermaltransformation state from its austenitic phase to its pearlite phase ina water-cooled jacket 26. This minimizes the time in which the materialis in its transformation stage and maximizes the temperature at whichthe material exits that stage and enters into the pearlite phase. In thepreferred embodiment, this occurs at approximately 950° F.

After this step is completed, the material is passed through an aircooling station 28 which can be comprised of a multiple fin-tubed heatexchanger. The tubing is maintained in a controlled atmosphere duringthe air cooling stage to prevent oxidation and discoloration of thefused copper material which would occur if it were exposed to oxygen.After the tubing reaches a temperature at or below 500° F., the materialmay be liquid quenched in the quench bath 30 to further reduce thelatent heat to a point where the continuous metal tube can be easilyhandled. The material can then be exposed to air and be subjected topost-process stations such as testing stations 32, stretching stations34, and eventually, a coiling station 36.

The process of the present invention permits the formation of uniquebrazed double-walled tubing from a copper plated strip. The tubing thusformed is comprised of a continuous latitudinal spiral of metalproviding two thicknesses of the non-ferritic steel at any point throughthe circumference of the tube. The edges of the copper-platednon-ferritic steel strip are suitably shaped to permit the edges tosealingly conform to the contours of the associated external or internalsurface of the tube. In this manner, the tubing can be brazed to providea seamless seal, as well as continuous brazing around 360° of thedouble-walled metal tube. This tube is uniquely constructed in that thematerial of choice is a non-ferritic steel, such as nickel chromiumsteel which is brazed by the action of heating copper material platedthereon. The resulting tube is highly resistant to corrosion.

What is claimed is:
 1. A process for brazing a selected metal alloy to anon-ferritic steel surface, the brazing process comprising the stepof:rapidly raising the temperature of the non-ferritic steel from afirst temperature essentially equivalent to the volatilizationtemperature of any lubricating materials adhering thereto to an elevatedbrazing temperature sufficient to trigger fusion between the selectedmetal alloy and the non-ferritic steel surface, said temperatureelevation occurring in a humidified gaseous atmosphere consistingessentially of a non-reactive carrier gas and a reactive gas suitablefor and in sufficient concentrations to achieve fluxing, wherein saidnon-ferritic steel is maintained in contact with said humidified gaseousatmosphere at said brazing temperature for an interval sufficient topermit fusion between the selected metal alloy and the non-ferriticsteel surface.
 2. The process of claim 1 further comprising the stepof:after metal fusion has been achieved, allowing resulting fused metalmaterial to cool to a first lowered temperature, said cooling occurringin a controlled non-oxidative atmosphere at a rate which maximizes thetemperature at which metallurgical transformation of the non-ferriticsteel from an austenitic to a pearlite phase occurs.
 3. The process ofclaim 2 further comprising the step of:rapidly cooling said fused metalin a controlled atmosphere to a temperature below which the selectedmetal alloy is not reactive with oxygen after reaching saidmetallurgical transformation point.
 4. The process of claim 1 whereinsaid humidified gaseous atmosphere suitable for achieving fluxingconsists essentially of nitrogen and sufficient hydrogen to achieve andmaintain fluxing, said humidified gaseous atmosphere and said controlledgaseous atmosphere having a dew point greater than about -42° F.
 5. Theprocess of claim 1 further comprising the step of depositing theselected metal alloy over the surface of the non-ferritic steel prior tobrazing.
 6. The process of claim 5 wherein the selected metal alloy iselectroplateable over the non-ferritic steel surface and said depositionstep is an electroplating process.
 7. The process of claim 1 wherein thenon-ferritic steel consists essentially of iron, chromium, nickel,manganese, silicon, nitrogen, and carbon.
 8. The process of claim 7wherein the non-ferritic steel contains no more than 0.03% by weightcarbon.
 9. The process of claim 3 further comprising the step ofquenching said fused metal in an aqueous medium after said fused metalhas cooled to a temperature below said non-oxidative temperature of theselected metal alloy.
 10. The process of claim 1 wherein saidlubricating materials overlay said surface of the non-ferritic steelwith the selected metal interposed therebetween, said lubricatingcompound consisting essentially of carbon and volatilizable solvents andcarriers, wherein said volatilizable solvents and carriers arevolatilizable at a temperature below about 900° F.
 11. The process ofclaim 10 further comprising the step of:elevating the temperature of thenon-ferritic steel from a preliminary temperature substantially belowsaid volatilization temperature to said volatilization temperature, saidtemperature elevation proceeding in the presence of a non-oxidativegaseous atmosphere at a rate sufficient to initiate an essentiallyinstantaneous volatilization of volatilizable solvents and carrierspresent in said lubricating material.
 12. A process for brazing at leasttwo nonferritic steel surfaces to one another, the brazing processcomprising the steps of:rapidly raising the temperature of thenon-ferritic steel surface from a first temperature to a brazingtemperature greater than about 2,000° F. at a rate sufficient toinitiate fusion between the non-ferritic steel surfaces and a selectedbrazeable metal deposited thereon in a humidified gaseous atmosphereconsisting essentially of nitrogen and sufficient hydrogen to permitmetal fluxing; and maintaining the non-ferritic steel surfaces and saidselected brazeable metal in contact with said humidified gaseousatmosphere at said brazing temperature for an interval sufficient topermit uniform fusion between said selected brazeable metal and thesurfaces of the non-ferritic steel.
 13. The process of claim 12 furthercomprising the steps of:after brazing has been achieved, maintainingsaid resulting fused metal material in said humidified gaseousatmosphere and allowing said fused metal material to cool to a firstlowered temperature greater than about 1,250° F.; upon reaching saidfirst lowered temperature, metallurgically freezing said brazed materialin the essentially crystalline configuration achieved at 1,250° F. byrapidly cooling said brazed material to a second lowered temperatureless than about 950° F., said cooling step occurring in said humidifiedgaseous atmosphere.
 14. The process of claim 13 further comprising thestep of:after reaching said second lowered temperature, cooling saidfused metal in a controlled atmosphere to a third lowered temperatureabout 500° F., below which said brazed material is not reactive withoxygen.
 15. The process of claim 12 further comprising the stepof:depositing a selected brazeable material on the respectivenon-ferritic steel surfaces to be brazed, said brazeable materialselected from the group consisting of silver, copper, silver alloys,copper alloys and mixtures thereof prior to brazing the non-ferriticsteel surfaces.
 16. The process of claim 15 further comprising the stepsof:applying lubricating materials to said material formed in claim 15,said lubricating materials consisting essentially of carbon and amixture of volatilizable solvents and carriers; and subjecting saidlubricated material to metal deformation processes.
 17. The process ofclaim 16 further comprising the steps of:instantaneously volatilizingsaid volatilizable solvents and carriers in said applied lubricatingmaterials by rapidly raising the surface temperature of said deformedmaterial from essentially ambient to a volatilization temperature lessthan about 800° F. to about 900° F., said volatilization step occurringin a controlled non-reactive gaseous atmosphere; and maintaining saiddeformed material at said volatilization temperature for an intervalsufficient to volatilize residual volatilizable solvent.
 18. The processof claim 13 wherein said hydrogen is present in an amount between about50% by volume and about 75% by volume of the gaseous mixture, and saidgaseous atmosphere has a dew point greater than about -42° F.
 19. Theprocess of claim 13 wherein the non-ferritic steel consists essentiallyof iron, chromium, nickel, manganese, silicon, nitrogen, and carbon. 20.A process for producing a non-corrosive, seamless double-walled tubingcomprising the steps of:layering a selected metal on the surface of asheet of non-ferritic metal, such that the selected metal overlays andis attached to said surface of the non-ferritic metal, the selectedmetal capable of forming a metallurgical bond with said surface of thenon-ferritic metal under subsequent controlled conditions; milling saidmetal sheet to sequentially form an unbrazed, unsealed continuousdouble-walled tube; removing volatile portions of lubricating materialemployed during said milling steps from said surface of said continuoustube, said volatile portions consisting essentially of solvents andcarriers; rapidly raising the surface temperature of said continuoustube to an elevated temperature sufficient to trigger fusion betweensaid non-ferritic steel and said selected metal layered there on, saidtemperature elevation occurring in a humidified gaseous atmosphere, saidhumidified gaseous atmosphere consisting essentially of at least oneinert carrier gas and sufficient hydrogen to achieve and maintainfluxing; maintaining said unsealed continuous tube in contact with saidhumidified gaseous atmosphere at said elevated temperature for aninterval sufficient to permit fusion between said selected metal andsaid non-ferritic steel, forming a fused leak-proof, double-walledcontinuous metal tube; after fusion between said selected metal and saidnon-ferritic metal has occurred, maintaining said resulting fuseddouble-walled metal tube in said humidified gaseous atmosphere andallowing said fused continuous metal tube to cool to temperature in afirst lowered temperature range, said first lowered temperature rangebeing lower than said fusion temperature and higher than the temperatureat which an initial metallurgical transformation of the non-ferriticsteel from an austenitic phase to an intermediate phase occurs; then,rapidly cooling said fused continuous metal tube to a temperature in asecond lowered temperature range, said second lowered temperature rangebeing lower than the temperature at which metallurgical transformationof said fused metal from said intermediate phase occurs; and afterreaching said temperature in said second lowered temperature range,slowly cooling said fused continuous metal tube in a non-oxidativeatmosphere to a third lowered temperature below which the selected metalfused on said continuous metal tube is not reactive with oxygen.
 21. Theprocess of claim 20 wherein said removal of volatile components of saidlubricating materials further comprises the steps of:elevating thetemperature of said unsealed continuous tube from an essentially ambienttemperature to a volatilization temperature, said volatilizationtemperature being sufficient to volatilize a substantial portion of saidsolvents and carriers present in said lubricating materials, saidelevating step proceeding in the presence of a nonoxidative gaseousatmosphere at a rate sufficient to initiate an essentially instantaneousvolatilization of said solvents and carriers present in said lubricatingmaterial.
 22. The process of claim 21 wherein said elevation of thetemperature of said continuous metal tube to said volatilizationtemperature occur essentially instantaneously.
 23. The process of claim21 wherein said volatilization temperature is between about 700° F. andabout 900° F.
 24. The process of claim 20 wherein said elevatedtemperature sufficient to trigger fusion between the nonferritic steeland the selected metal layered thereon is greater than the melting pointof the selected metal.
 25. The process of claim 20 wherein said elevatedtemperature sufficient to trigger fusion between the nonferritic steeland the selected metal layered thereon is greater than about 2,050° F.26. The process of claim 20 wherein said second lowered temperature isless than about 950° F.
 27. The process of claim 20 wherein saidhydrogen is present in an amount between about 50% by volume and about75% by volume of the gaseous mixture and said gaseous atmospheresuitable for maintaining fluxing has a dew point greater than about -42°F.
 28. The process of claim 20 wherein the metal capable of beingelectroplateable over non-ferritic steel is selected from the groupconsisting of copper, silver, copper alloys, silver alloys, and mixturesthereof.
 29. The process of claim 28 wherein the non-ferritic steelconsists essentially of iron, chromium, nickel, manganese, silicon,nitrogen, and carbon.
 30. The process of claim 29 wherein thenon-ferritic steel contains no more than 0.03% by weight carbon.
 31. Theprocess of claim 30 wherein said third lowered temperature is less thanabout 500° F.
 32. The process of claim 31 further comprising the stepof:exposing the non-ferritic steel to ambient temperature prior toraising the surface temperature of the non-ferritic material to saidelevated temperature, said exposure occurring for an interval sufficientto permit removal of said non-oxidative gaseous atmosphere.